1. Field of the Invention
The present invention relates to ovens, especially for cooking, of the individual type, for example for the home, of the collective type, for example for a school or company canteen, or of the industrial type, for preparing cooked dishes in the agrofoodstuffs field.
2. Description of the Prior Art
Because of the great changes in appearance and taste that they can suffer, cooking some foods raises difficult problems of adjusting the temperature and controlling cooking. Red meat, such as roast beef, is one of the most difficult foods to cook. The user or the consumer may require the meat to be cooked rare, medium rare or well done. The core temperature difference of the beef for these three types of cooking is of the order of 1 to 2xc2x0 C.
xe2x80x9cConventionalxe2x80x9d dry cooking and steaming at a high temperature very much higher than the required core temperature within the product are known in the art. Adjusting the oven is a simple matter: a temperature and a time are decided on. A temperature probe stuck into the beef can help to determine when it is cooked. The cooking time is short. The beef has the required external appearance, with a center that is more or less red or pink and a lightly browned outer crust. It can be sliced and served directly, with no subsequent operation.
However, it is very difficult to control the degree of cooking correctly. The high temperature difference between the middle of the beef and its surface generates a high inertia, which must be estimated accurately in order to remove the beef from the oven before the target core temperature is reached and allow it to xe2x80x9crestxe2x80x9d outside the oven, in the hope that the thermal inertia will enable it to reach approximately the required core temperature, and then carve it at the proper time. There may be significant shrinkage through loss of weight. The tenderness and the juiciness of the beef are not optimum, as the joint can harden and lose juice during carving. Two different size joints of beef cannot be cooked to the same degree and must therefore be removed from the oven several minutes apart, which is particularly bothersome in the context of a collective kitchen. Also, a joint of beef of average quality will also have an average quality appearance and taste after cooking.
The resting time outside the oven after cooking and during which the temperature becomes more uniform is hardly satisfactory from the sanitary point of view, the beef being exposed to dirt and to external pollution, and the meat may be allowed to rest for too long. Cooking at high temperatures often causes significant splashing of grease onto the walls of the oven. The splashes are carbonized because of the high temperature and the dry atmosphere. Thorough and therefore time-consuming and labor-intensive cleaning is necessary to keep the oven clean. Furthermore, a person must remain nearby during the cooking phases to control and monitor the cooking process. Thus the oven can be used only when staff are present. Energy is consumed shortly before the meal, i.e. in the middle of the day, at a time when electrical energy costs are high.
Cooking in steam at a temperature only slightly greater (by 2 to 3xc2x0 C.) than the required core temperature is also known in the art. This ensures accurate control of the degree of cooking. The end of cooking time does not need to be particularly precise, given that the temperature inside the oven is substantially equal to the required core temperature. It is therefore not necessary to provide any resting time on removing the beef from the oven. Its appearance after carving is properly controlled. The gustatory, tenderness and juiciness aspects are very well preserved regardless of the size of the joint of beef and the load in the oven, i.e. the number of joints of beef cooked at the same time. Bacteria in the meat are thoroughly destroyed because of the long cooking time. The loss of weight of the beef is greatly reduced.
However, cooking takes several hours, which immobilizes an oven for longer. The beef does not have the browned external appearance that the consumer may require. It may therefore be subjected to subsequent treatment with browning agents, sauces, etc. This type of cooking does not necessitate sophisticated resources, except for very accurate temperature regulation. A bain-marie whose water temperature is closely regulated can be used for this type of cooking, for example.
The present invention seeks to combine the advantages of the two types of cooking described hereinabove and to avoid their drawbacks.
The present invention proposes an oven equally suitable for fast cooking and for slow cooking, which can be effected overnight and in the absence of staff.
The oven according to the invention includes an enclosure defining an interior space, means for heating the interior space, means for controlling the heating means, and means for cooling the interior space from a particular first temperature to a particular second temperature lower than the first temperature.
This means that successive cooking phases can be carried out at precisely defined temperatures to obtain roast beef having a browned external appearance thanks to a first phase conducted at a high temperature, followed by a cooling phase to cool the outside areas of the beef and prevent them transmitting excessive heat to the core of the meat, and a temperature maintenance phase during which the temperature of the various areas of the product becomes more uniform, the degree of cooking then being determined not by the duration of the temperature maintenance phase but only by the temperature.
In one embodiment of the invention the cooling means are controlled by the control means.
The cooling means advantageously include means for evacuating hot air contained in the interior space and means for introducing cold air into the interior space.
More particularly, the cooling means can include an air pump and a valve disposed between the pump and the interior space in the direction of flow of the air, closed when the pump is stopped and open when the pump is operating. The valve prevents unwanted circulation of air during the cooling phase. The pump can be a centrifugal pump.
In one embodiment of the invention the cooling means include means for injecting water. Thus the temperature of the heating means, for example electrical heating elements, can be reduced very quickly by the high calorific capacity of the water. It is therefore particularly beneficial to spray water onto an electrical heating element. Water can be sprayed by means of a fan and can also be directed toward the walls of the interior space, which have some thermal inertia because of their thickness and the insulating materials with which they are surrounded.
The method of controlling an oven according to one aspect of the invention initiates a step of forced cooling of the interior space of an oven initially at a first temperature. Cooling is continued to a second temperature lower than the first temperature.
Cooling air is advantageously drawn in from an external environment, for example the surrounding air. The second temperature is higher than the ambient temperature around the oven.
The method advantageously includes a first step of heating the interior space to a first temperature, a second step of forced cooling to a second temperature, and a third step of maintaining the second temperature.
Furthermore the method can include a fourth step of forced cooling to a third temperature lower than the second temperature and a fifth step of maintaining the third temperature. Other similar steps toward increasingly lower temperatures can be used.
The humidity in the interior space is preferably estimated and controlled.
In one embodiment of the invention the forced cooling step includes spraying water inside the oven, which is particularly effective if the temperature in the interior space is greater than 100xc2x0 C., and circulation of air with entry of cold air and exit of hot air, which is more efficient for temperatures less than 100xc2x0 C.
The invention also proposes a method of controlling an oven in which means for heating the interior space of the oven are adapted to operate at a number n of non-zero heating powers. A temperature Te of the interior space is measured and if Te less than Tcxe2x88x92BP1, where Tc is a set point temperature and BP1 is a temperature range, then the heating power P is adjusted to the maximum power Pmax. If the temperature Te is in the range from Tcxe2x88x92BPi to Tcxe2x88x92BPi+1 where i is in the range from 1 to n, then P is equal to j/nxc3x97Pmax. If the temperature Te is higher than the set point temperature then P=0 and BPi=xcex1ixc3x97p+xcex2i, where p is the temperature rise slope in degrees/second, xcex1 is a constant expressed in seconds, and xcex2 is a constant expressed in degrees. If the temperature Te crosses the value Tcxe2x88x92BPi on increasing and if the slope p becomes negative, the power P is increased by 1/nxc3x97Pmax. If Te greater than Tc+1xc2x0 C., then the power P is decremented by 1/n Pmax, if not j=nxe2x88x92i.
In one embodiment of the invention 2xe2x89xa6nxe2x89xa65. For example, n=3.
In one embodiment of the invention xcex1i=1/(n+1), in other words BPi=p/(n+1)+xcex2i.
In one embodiment of the invention BP1=p/4+10xc2x0 C. and BP2=p/4+2xc2x0 C.
The slope is advantageously calculated for each 0.5xc2x0 C. variation in temperature. Thus p=0.5/xcex94t1, xcex94t1 being the time period during which the temperature varies by 0.5xc2x0 C. If xcex94t1 is less than 3 seconds, then p is calculated for a temperature variation of 1xc2x0 C. with P=1/xcex94t2, xcex94t2 being the time period during which the temperature varies by 1xc2x0 C. If xcex94t2 is less than 3 seconds, then p is equal to 2/xcex94t3, xcex94t3 being the time period during which the temperature varies by 2xc2x0 C. If xcex94t3 is less than 3 seconds, then p is equal to 4/xcex94t4 where xcex94t4 is the time period during which the temperature varies by 4xc2x0 C.Q. In this way it is possible to avoid reacting to transient phenomena causing erroneous commands to be sent to the oven.
The principle of cooking by temperature phases of particular duration enables the chaining of browning, core cooking and, where applicable, maintenance phases with complete control of all parameters for obtaining optimum quality and achieving the advantages of slow cooking in a short time. The first phase of cooking at high temperature forms a crust that limits loss of weight and has the usual appearance of roast beef. Cooling prevents the exterior areas of the beef transmitting excessive heat to the core area. The temperature maintenance phase determines the core cooking temperature extremely precisely. The product can remain in the temperature maintenance phase for a widely varying time, given a temperature equivalent to the required core temperature and the absence of harmful effects of heat transfer between a hotter area and a cooler area and the absence of the resting time necessary for obtaining more uniform temperatures. The tenderness and the juiciness are optimum. The gustatory aspects are preserved, independently of the size of the product and the load of the oven. The pasteurizing effect is high, which guarantees satisfactory destruction of bacteria. Soiling of the oven is reduced and the low-temperature steam cooking phase prevents the carbonization of splashes and their adherence to the walls of the oven. This reduces the weight loss of the product to be cooked and reduces the time and therefore the cost of cleaning the oven after cooking.
An overnight cooking program can also be provided. The products are set to cook in the evening before the staff go home and are taken out of the oven on the next day when they are to be served. The electrical power consumed by the oven is then charged at the overnight rate, which is generally cheap. The oven is used in masked time, when no staff are present, which facilitates organizing a short shift. The oven is operated outside the working hours of the staff for better amortization. The products can be taken out of the oven as soon as staff arrive in the morning. The oven is then available for cooking other products.
The present invention will be better understood and other advantages will become apparent on reading the detailed description of a few embodiments provided by way of non-limiting example and shown in the accompanying drawings.